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https://issues.apache.org/jira/browse/CASSANDRA-7486?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14523621#comment-14523621
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Benedict commented on CASSANDRA-7486:
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bq. ^ G1 doesn't work that way.
While it has no "old" generation, it does promote regions and if this happens a
lot you can get some weird pathological fragmentation. Now, my experience with
G1 is out of date, and I haven't kept up at all with its latest behaviours, but
I saw some really atrocious behaviour on very simple benchmarks a few years
back. At the time, If you modified references that were randomly distributed
around the heap, it required traversing a majority of the heap to collect very
little, and essentially thrashed. I realise it has improved, but I do not know
in what ways, and so I'm wary of making it the default without being certain it
no longer has pathological cases that are a problem for us. Unless we stress
the collector so that it exercises its suboptimal characteristics, I am not
really super confident. I hope this is simply overly cautious, but we know of
users who also had serious problems with sudden degradation despite looking
good in initial testing, and it would be great for that not to be a widespread
problem.
> Compare CMS and G1 pause times
> ------------------------------
>
> Key: CASSANDRA-7486
> URL: https://issues.apache.org/jira/browse/CASSANDRA-7486
> Project: Cassandra
> Issue Type: Test
> Components: Config
> Reporter: Jonathan Ellis
> Assignee: Shawn Kumar
> Fix For: 2.1.x
>
>
> See
> http://www.slideshare.net/MonicaBeckwith/garbage-first-garbage-collector-g1-7486gc-migration-to-expectations-and-advanced-tuning
> and https://twitter.com/rbranson/status/482113561431265281
> May want to default 2.1 to G1.
> 2.1 is a different animal from 2.0 after moving most of memtables off heap.
> Suspect this will help G1 even more than CMS. (NB this is off by default but
> needs to be part of the test.)
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